Impressions are obtained by means of impression trays having different configurations according to the kind of impression to be taken, i.e. suitable either for the impression of the complete upper or lower dental arcade or for a section of the upper or lower dental arcade.
The impression trays are generally made of metal, namely aluminium or stainless steel or plastic and have a "U"-shaped section. The impression trays are filled with plastic material, and then they are introduced into the mouth and pressed down on the dental arcade whose impression has to be taken. Thus the plastic material is impressed and takes the configuration of the dental arcade.
When the impression has to be taken for both the upper arcade and lower arcade two single separate impressions are needed with a consequent loss of time and the risk of jeopardizing their precision when coupling both parts together. Usually, the impression tray is maintained in position though a finger pressure by the patient or by the dentist till the plastic material filling it has set. However, such a method is not safe as the pressure exerted by the fingers is not steady over the whole impression tray during the setting time of the impressing material and not be strong enough so that the result of the operation may be jeopardized.
After the plastic material has set, the impression tray is removed from the patient's mouth by pulling it off the dental arcade with manual movements which, in turn may cause deformations to the impression, jeopardizing the subsequent work.
Moreover, to avoid that the plastic material may remain stuck to the dental wall when removing the impression tray, impression trays are provided with a set of holes arranged on several rows, both on the side walls and on the bottom of the "U"-shaped section, over the whole extension of the impression trays Thus, the plastic material in the impression tray exceeding the quantity required for the purposes of the impression can run out of the holes and cause a retention along with the impression tray.
Therefore, this method requires the use of more plastic material than actually needed for the impression alone, with consequent higher costs.